1. Field of the Invention
This invention relates to heat exchangers, for example, the heat exchangers of nuclear steam generators. In particular, the invention relates to baffle plates for such heat exchangers and techniques for forming the tube-receiving openings in such plates.
2. Description of the Prior Art
In one type of nuclear steam generator, primary coolant from a nuclear reactor flows through a bundle of inverted U-shaped tubes, which have their lower ends received in a tube sheet. A secondary coolant flows around the tubes in heat exchange relationship therewith. More particularly, the secondary coolant generally flows downwardly in an annulus along the outer periphery of the tube bundle to the tube sheet and then upwardly among the tubes. Particles of sludge tend to settle out from the secondary coolant and build up along the top of the tube sheet. These piles of sludge deposits contribute to corrosion of the tubes.
In order to minimize the buildup of sludge on the tube sheet, it is known to use a baffle plate a slight distance above the tube sheet to redirect a portion of the flow of the secondary coolant. More specifically, such a baffle plate is designed to impede the upward flow velocity and, therefore, tend to increase the laterally or radially inwardly directed flow velocity along the tube sheet. This increased lateral flow velocity tends to entrain sludge particles and prevent them from depositing on the tube sheet. The baffle plate has a plurality of openings therethrough for respectively accommodating the tubes, these openings being greater in diameter than the tubes to permit fluid flow through the openings along the outsides of the tubes. However, the tolerances are such that generally the tubes are not aligned exactly coaxially with the tube openings in the baffle plate.
The portion of the tube disposed in the tube opening co-operates with the surrounding portion of the baffle plate to define a generally annular flow channel, which is a volume bounded on the inside by the tube and on the outside by the perimeter of the tube opening in the baffle, and on the top and bottom by the planes of the upper and lower surfaces of the baffle plate. Typically one side of a tube will contact the baffle plate and, immediately on either side of the contact region, the cross-sectional area of the flow channel around the tube is insufficiently large to prevent the secondary coolant liquid which enters the channel from completely boiling dry before exiting the channel. This boiling condition results in deposition of sludge and corrodents in these crevice-shaped portions of the flow channels, which may lead to tube corrosion at these locations, a condition which is sometimes referred to as "crevice corrosion".
In order to minimize such crevice corrosion, it is known to provide the baffle plate with non-circular tube-receiving openings. One such arrangement utilizes an opening which has frustoconical portions at the upper and lower surfaces of the tube sheet which converge inwardly to an octagonal portion centrally of the baffle plate thickness. The tube can contact the baffle plate only along the flat sides of the octagonal portion of the opening. These flat sides tend to provide crevices on the opposite sides of the contact point which are wide enough to inhibit crevice corrosion. However, because of the thin axial extent of the octagonal portion of the opening, there is a tendency to produce excessive wear on the outer surface of the tube at the contact points.
An alternative arrangement which has been used is tube-receiving openings which are generally multi-lobed in shape, each opening having a plurality of radially inwardly directing protrusions which are designed to provide generally line contact with the associated tube. The lobed portions of the opening are designed so as to eliminate crevices and provide a substantial flow area so as to overcome crevice corrosion.
Such lobed tube openings have been relatively successful in minimizing crevice corrosion, but the flow volume through the tube openings is so great as to be of minimal use in redirecting the flow, i.e., in performing as a baffle plate.